Tri-clamp power pistons and crank linkage: full-bore ring-less pistons, con-rods, crank webs and a clamp-on crank frame for 1.5, 2 and 3 in spools — dimension sheet Source: Xem Studio, https://xemstudio.com/objects/tri-clamp-power-pistons/ (CC BY 4.0) Basis: 16 ga ASME BPE tube bores (1.5 in = 34.8 mm, 2 in = 47.5, 3 in = 72.9); finished ring-less piston fit 0.02–0.05 mm diametral (graphite-in-glass and lapped-aluminium Stirling practice); FDM print-and-lap offsets from the Xem Studio calibration coupons Catalog version: 2026-10-20 Sizes: see table below - A Stirling power piston has no ring and no O-ring: the seal is the fit. Finished diametral clearance 0.02–0.05 mm; the test is that the piston falls slowly through the spool under its own weight with the far end open and holds for several seconds with it closed. - You cannot print that fit. Print the closest offset your printer makes (f010/f020/f030 = 0.10/0.20/0.30 mm under bore after the hole/pin coupons have set your slicer), or the 'lap' version 0.10 mm over, then lap it in: 800 → 1500 grit wet, then metal polish, turning the piston in the actual spool on the lapping handle. PETG laps well; PLA galls. Finish with a dry graphite rub (pencil) and run it dry; oil adds friction and stiction in a Stirling. - Length 1.2 × bore keeps it from cocking: the two 8 mm lands at the ends carry the fit; the 0.4 mm relief between them halves the rubbing area and traps dust. Wrist pin Ø3 (dowel-pins-metric) at 0.42 L, con-rod small end with a 623 bearing from the crank kit; the skirt is open so the rod swings inside it. - Where it goes: beta engine (piston and displacer in one spool, displacer rod through the piston's wrist-pin gap), alpha (two spools at 90°, a piston in each, no displacer), or a big low-compression gamma (same-bore power cylinder: 1:1 swept ratio, suits an LTD on a hot plate). For a high-ratio gamma use the Ø20 piston from the power-cylinder entry instead. - The -rod variants (f020 and lap) have a Ø5 crown boss for a dry 3 × 5 × 6 bushing (graphite-bronze, PTFE, or sintered bronze degreased in acetone; no oil) so a Ø3 displacer rod runs through the piston on a clearance fit: that is the beta / rhombic / free-piston piston. The piston plug in the tri-clamp linkage kit drops into the skirt, is retained by the two wrist-pin dowels, and carries the 5 mm tube piston rod with the displacer rod inside it. - Mass: the 2 in piston prints at ~21 g in PETG (16.8 cm³); keep it light, it is the moving mass the flywheel has to swing. - Linkage, designed so nothing else is needed: the crank frame is an ISO 2852 cap with a con-rod window and two bearing towers, so a stock clamp holds it on the spool's open ferrule; a Ø8 shaft (shafts entry, 80–100 mm) runs in two 608s; two crank webs (Ø8 bore, M3 grub) carry a Ø3 pin at stroke/2 between them; the con-rod runs on 623s at both ends. Order along the shaft: tower · web · rod · web · tower, 1 mm gaps. The flywheel (Ø80–100, Ø8 bore, crank kit) and an 8 mm collar go outside one tower. - Geometry: piston skirt flush with the ferrule face at bottom dead centre, so the lands never leave the bore; rod centres = wrist-pin height + crank height + stroke/2 (1.5 in: 58, 2 in: 70, 3 in: 95); rod angle under 11°. Stroke 16 / 24 / 36 mm = 0.46 × bore, which gives a 2 in alpha or beta ~42 mL swept per cylinder. - Wrist pin: Ø3 rod cut to the length in the table (drill rod, music wire or a 3 mm dowel for 1.5 in); it is retained by the spool bore, no clips. Set the webs 180° apart on an alpha's second cylinder, or add the counterweighted cranks entry's webs for balance. - LES: the piston the tri-clamp beta and alpha engines run in a stock spool with no liner. spool bore_mm piston_od_mm length_mm offset_mm wrist_pin_z_mm 1.5 34.800000000000004 34.9 42 +0.10 17.6 1.5 34.800000000000004 34.7 42 -0.10 17.6 1.5 34.800000000000004 34.6 42 -0.20 17.6 1.5 34.800000000000004 34.5 42 -0.30 17.6 2 47.5 47.6 56 +0.10 23.5 2 47.5 47.4 56 -0.10 23.5 2 47.5 47.3 56 -0.20 23.5 2 47.5 47.2 56 -0.30 23.5 3 72.9 73.0 88 +0.10 37.0 3 72.9 72.8 88 -0.10 37.0 3 72.9 72.7 88 -0.20 37.0 3 72.9 72.6 88 -0.30 37.0 Files elsewhere (not hosted by Xem Studio; license as stated by the publisher): - Long Earth Society: Tri-clamp Stirling engine builds — web, video — LES site — https://longearthsociety.com/engineering/ - James R. Senft: An Introduction to Stirling Engines (piston fit and leakage) — book — Moriya Press [verify license at link] — https://www.stirlingengine.com/ - ASME: BPE tube dimensions (16 ga sanitary tube) — standard — ASME [verify license at link] — https://www.asme.org/codes-standards/find-codes-standards/bpe-bioprocessing-equipment Free files (CC BY 4.0, Xem Studio; STEP + STL, 30 parts): https://xemstudio.com/files/hardware/tri-clamp-power-pistons/index.json API: https://xemstudio.com/api/geometry?object=tri-clamp-power-pistons